gitk: Fix Update menu item

gitk: Fix Update menu item

This just does the simple thing of resetting everything, reading all
the commits, and redoing the whole layout from scratch.  Hopefully
things are now fast enough that this simple approach is acceptable.
Also, this fits in better with future plans for adding the ability
to restrict the tree to just a few files and then expand back to
the whole tree.

Signed-off-by: Paul Mackerras <paulus@samba.org>